1. Field of the Invention
This invention generally relates to an intermediate resin blend that can be used to formulate improved coatings for various substrates and more particularly to an intermediate resin blend that can be used to manufacture a final coating that provides water, abrasion, chemical and hot tire pick up resistance while minimizing the possibility of osmotic blistering of the coating.
2. Description of Related Art
Various polymer coatings and sealants are often applied to substrates such as wood, cement, concrete, stone, stucco and cement-based substrates in order to provide beneficial properties to the substrate. One such quality often desired in a coating is the ability to provide a waterproof or water resistant coating. Other desirable characteristics of a coating or sealer is include one that is highly resistant to different types of chemicals as well as scratch and mar resistance.
One conventional method of providing water resistance to a substrate is to use a penetrating sealer. These types of sealers penetrate into the substrate to seal the surface. While providing some measure of water resistance, since these sealers do not form a film on the surface of the substrate, they are generally not able to confer much chemical and abrasion resistance. Nor are they particularly useful to change the appearance of the underlying substrate.
Another conventional method of providing the desired water resistance, in particular to a cement or concrete substrate, is to use an impermeable sealant coating that forms a film on the surface of the substrate. One such impermeable sealant is a 2-part epoxy coating. This coating uses epoxy chemistry to provide a low or no permeable coating that isolates the concrete substrate. The epoxy sealant may be used as a primer below another coating or can be tinted or used alone as a final coating.
One drawback of using an impermeable sealant coating such as a 2-part epoxy is that the coating is subject to osmotic blistering and delamination. As an initial matter, to the extent the coating is applied to a concrete slab that is on the ground and at a grade that is below the water table, there will be hydrostatic pressure against the underside of the coating that would tend to create blisters and/delaminate the coating from the concrete substrate. However, even if the concrete slab is above the water table or even above grade, it can still be subject to osmotic blistering.
Osmotic pressure is the spontaneous flow of water through a semi-permeable membrane from a dilute solution to a more concentrated solution of salts or water-soluble organics. As a result, the liquid volume of the initially more concentrated side increases until it generates a hydrostatic pressure that is equivalent to the remaining osmotic pressure. This osmotic pressure can greatly exceed the other forces present in concrete and are several orders of magnitude greater than the hydrostatic forces observed in uncoated concrete. Some reports indicate that the osmotic pressure can be as high as 3000 psi, which is much greater than the bond of epoxies and other coatings to a concrete substrate. Currently there is no test available to determine if osmotic blistering will occur in a given situation. Osmotic blistering can occur even when the moisture vapor emanating out of a 100 square foot section in 24 hour is less than 3 pounds of moisture vapor, which is considered by the construction industry to be the maximum amount permitted to emanate from a concrete slab that is to be coated or covered with an impermeable coating or flooring.
The concrete slab itself can act as semi-permeable membrane by allowing the passage of water but not larger water soluble materials. The pore size found in good quality concrete can be a suitable size (1-2*10-5 mm) to form a semi-permeable membrane. Even if the pore size is not small enough, application of a primer coat can often reduce the pore diameter sufficient for it to act as a semi-permeable membrane. The presence of water and salts or water soluble organics are typically found in concrete. Even when considered completely dry, a concrete slab will still contain up to 5% free water by weight and on average measures roughly 70% insitu relative humidity. When the concrete slab is in contact with the ground, additional water may be present from the migration of water vapor up through the slab.
The water soluble materials can be anything from the resin ingredients in the epoxy coating, materials from acid etching the concrete before coating, sodium/potassium silicates, sulfates resulting from the sulfur trioxide used in the concrete, hydrolysis products, salts present in the water or aggregate used to make the concrete, or unreacted paste. In fact, it has been known that there can be a significant increase in the soluble salt concentration at the surface of a concrete slab from which moisture is evaporating. When any water soluble materials form in a pocket between the concrete and the coating it can create a high concentration solution that draws additional water through the concrete slab thus forming osmotic blistering and delamination of the coating.
A pocket between the concrete slab and the coating can form from pre-existing pits in the concrete surface that are not adequately filled by the coating when applied. Alternatively, pockets can form as a result of hydrolysis. Corrosive hydrolytic attack can destroy chemical bonds in the sealant film that both results in the formation of a pocket as well as the necessary water soluble material to form a high concentration solution. In some cases, water in the concrete itself can exert sufficient pressure to debond a portion of the resin, especially if it is not yet fully cured, thus forming one or more pockets.
Depending on the particular context, the presence of osmotic blistering and/or delamination of the coating can range the gamut from being merely a cosmetic nuisance all the way to being a major health code violation. For example, food processing facilities are required by law and/or regulation to have their floors sealed in order to prevent microbial growth. This would mean that the presence of delamination and osmotic blistering in the coating would require that the coating be immediately stripped and the concrete floor recoated. In addition to the added cost of recoating the concrete flooring, this could result in significant additional costs due to the unscheduled facility downtime required to conduct such recoating.
There have been some attempts at addressing the problem of osmotic blistering. However, high water resistance is generally considered mutually exclusive to a high vapor passage through the film. As a result, these attempts have generally used a two layer approach where a first layer or primer attempts to minimize and/or prevent the buildup of osmotic pressure and a second topcoat layer, typically of a 2-part epoxy is used to provide the desired water resistance and other properties.
In addition to the osmotic blistering issue when trying to provide a water resistant coating or sealant, there are a number of other characteristics that are important for a coating or sealant to possess. Where the coated substrate will be subject to vehicular traffic, another concern for the polymer coating is to avoid hot tire pickup. When a vehicle has been driven for a sufficient period that its tires are hot and then parked on the coated substrate for a period of time. At this point the entire weight of the vehicle is dispersed over the relatively small surface area of the tires that are actually in contact with the coated floor. Especially since many coatings are thermoplastic in nature, the heat and pressure generated between the tire and the coated floor it is in contact with can be sufficient to delaminate the polymer coating. This results in the coating peeling off the substrate when the vehicle is next moved.
Current and proposed environmental regulations create additional challenges in addressing these issues. Current regulations limit the amount of volatile organic compounds (VOC) that are permissible in coatings as well as limiting the potential solvents or coalescent used in the coating product. The regulatory trend is to further reduce the permissible VOC levels and the particular solvents that can be used. The allowable level of VOCs as well as the particular solvents that can be used can vary based on the particular jurisdiction in which the final coating is ultimately sold.
Many of the low VOC coatings currently available achieve their low VOC values by using soft emulsion particles. Unfortunately, this generally leads to a soft film, which is not desirable for coatings used on certain substrates such as concrete. Harder coatings in general are desired for sealers as they will withstand abrasion much better as long as it is no so hard that it becomes brittle and can shatter if a heavy object were dropped on the coated substrate. If the substrate is wood or another substrate that have some natural structural movement the coating also needs to be flexible enough to move with the underlying structural movement of the substrate.
A coating's hardness can usually be predicted by the glass transition temperature (Tg) of the resin used to make the coating. A resin with a higher Tg would result in a harder coating. An old rule of thumb is that the amount of coalescing solvent required to form a fully coalesced film is determined by taking the Tg temperature of the resign in ° C. and dividing it by two. This yields the number of parts of solvent required per 100 parts of resin solids. Thus, lower VOC emulsion resin based coatings were generally achieved by using resins with lower Tg temperatures because it reduced the amount of coalescing solvent that is required. However, this also results in a softer overall coating, which is often less desirable. In contrast, resins with higher Tg values that result in harder film coatings typically require higher levels of cosolvent to coalesce, rendering it more difficult or impossible to meet current VOC requirements as well as expected future reductions in permissible VOC levels.
For a floor coating, it is also important for the final coating to have a coefficient of friction that falls within a desired range. For example, in the U.S. the 1990 American with Disabilities Act (ADA) as well as the U.S. Occupational Health and Safety Administration (OSHA) recommends a coefficient of friction (C.O.F.) rating of from 0.5 to 0.8. A lower coefficient of friction would result in a surface that is too slippery and could results in individuals slipping and losing footing on the coated substrate. In contrast, too high a coefficient of friction can result in too much adhesion and the impression that an individual's feet are “sticking” to the floor.
Thus, there remains a need for improved resin blends that can be used to form coatings or sealants on various substrates such as wood, cement, concrete, stone, stucco and cement-based substrates in order to provide beneficial properties to the substrate.